I have seen some observations on the interwebs that some AAC and MP3 files are not displaying their artwork metadata as the file’s icon under High Sierra 10.13.1. That is: the file for a track with assigned artwork in iTunes displays in the Finder with a generic audio icon rather than the album artwork. If I recall correctly, there have been periods over the years where this has both worked and not worked as expected.
I’ve paid little attention to this issue because I don’t care about artwork for my audio files. Assigned artwork for tracks in iTunes is one thing, but how the files appear in the Finder is of little interest to me. But obviously this is something that many users have come to expect if not rely on.
My understanding is that Apple is aware of this issue. In fact, I’m running the latest High Sierra 10.13.2 beta and don’t detect a problem. So don’t sweat it.
UPDATE, December 6, 2017: This issue appears resolved after installing macOS 10.13.2.